基本 (jī běn) as an adjective means 'basic, fundamental' — what is essential or foundational, whereas 主要 (zhǔ yào) means 'main' (primary among options) and 根本 (gēn běn) is stronger, often used in negatives ('at all'). As an adverb it means 'basically, essentially', more formal than 基本上 (jī běn shàng), which can also mean 'on the whole'.